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I have modified a particular file (A LaTeX Beamer Template)

The readme says:

  1. The code of the package is dual-license. This means that you can decide which license you wish to use when using the beamer package. The two options are:

    a) You can use the GNU General Public License, Version 2 or any later version published by the Free Software Foundation.

    b) You can use the LaTeX Project Public License, version 1.3c or (at your option) any later version.

I can't seem to find the license files I expect to be included, though.

I might like to just license my derived work under only the The LaTeX Project Public Li­cense.

Can I do so? Or must I license it under both?

3

The key here is that you can use either license.

you can decide which license

That means that you can either redistribute according to the terms of the GPL, or the LaTeX license.

Dual license is generally used when the authors want to provide more licensing and distribution options than one license provides (in which case requiring use under both terms would be nonsensical), or when two licenses are incompatible (in which case tequiring use under both terms would be impossible).

2

Dual licensing is normally used when the licenses are not compatible. In such case, demanding to match requirements of both licenses at the same time means that the program cannot be redistributed at all. Even if you yourself are capable of following requirements of both licenses, you cannot give your users permissions to do that one license allows when another disallows.

One license gives you permission and restriction set and another license gives you another permission and restriction set; you can choose the permission set you prefer.

Of course, this is true if both licenses apply strictly to the same content. If the package has part of content under one license and part under another, both licenses apply at the same time. In such case the licenses must be compatible, otherwise the combination cannot be legally distributed. The most typical case of this collision is an attempt to combine a GPL-licensed part with closed source part (not possible).

1

Without seeing the original licence grant it's hard to be sure; the devil is in that "and/or". What you've written suggests that you may choose to use (run/distribute/modify/distribute-modified) the work under either of two licences, at your discretion.

Edit: with the clarifications you have made, this appears to me to be the case. Choose to receive the work under the LPPL, and release your modified version under that. (In the interests of free software, I urge you to receive it under the GPL, and publish your modified version under that accordingly; but that is, of course, up to you.)

If you decide to accept it under the LPPL, then I think you will be fine to redistribute your modified version under that licence, provided you meet the relevant conditions (see, eg, s.6).

If you decide to accept and use it under the GPL, you may not. GNU GPL v2 says in s.2:

But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License

GNU GPL v3 has a similar provision, in s.5(c):

You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy. This License will therefore apply, along with any applicable section 7 additional terms, to the whole of the work, and all its parts, regardless of how they are packaged.

  • You answered the question, no reason to push your personal favorite ---virus--- license. – RubberDuck Jan 10 '16 at 2:19
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    @RubberDuck the GPL is no more viral than any proprietary licence. So, and since this is an SE site specifically devoted to FLOSS, I will continue thus to express myself in my answers. You should of course feel free - as you have - to comment, and to downvote, but I will continue to express this opinion in my answers until a clear consensus that it is inappropriate emerges. – MadHatter supports Monica Jan 10 '16 at 7:58
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Unless your changes are wide-reaching, or you have a solid reason (i.e., legal/technical, not just "I dislike the Foo license"), common courtesy suggests to keep the wishes of the authors, i.e., both licenses.

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